Alloimmunization to HLA antigens and resultant refractoriness to random platelet transfusions occurs in 30 to 90% of patients undergoing treatment for acute leukemia and aplastic anemia. Management of the alloimmunized patient is difficult and consists mainly in provision of HLA-matched platelet components. In some cases, however, even transfusion with "HLA- matched" platelets will not effectively raise the platelet count, placing the patient at major risk of spontaneous bleeding. Recent reports of the successful removal of platelet autoantibody in patients with autoimmune thrombocytopenic purpura by staphylococcal protein A column absorption suggested that this procedure might be of benefit in the removal of platelet alloantibody in patients refractory to platelet transfusions. This study was designed to determine if staphylococcal protein A absorption is an effective therapy for platelet alloimmunization. Fifteen patients refractory to platelet transfusions due to HLA or platelet specific alloantibodies will undergo four staph protein A apheresis treatments over a two week period. Response to random-donor and HLA-matched platelet concentrates and pattern of reactivity of HLA antibodies will be assessed before and after protein A treatments. To date, only one patient has completed the protocol. No improvement in this patient's response to platelet transfusions was seen after protein A absorption. No adverse effects of therapy were encountered. This study continues to accrue patients.